JP3287214B2 - Multi-channel optical frequency stabilizer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used as a light source for optical frequency multiplexing (or wavelength multiplexing) communication. In particular, it relates to stabilization of an oscillation frequency (oscillation wavelength).

[0002]

2. Description of the Related Art Optical frequency multiplexing (OFDM)
uency Division Multiplexing, wavelength multiplexing, WDM: Wa
velength Frequency Division Multiplexing) The communication system multiplexes and transmits signals from multiple light sources with different oscillation frequencies on the transmitting side, and separates the signals for each optical frequency by an optical demultiplexer or optical filter on the receiving side. It receives and demodulates each channel. At this time, if the oscillation frequency fluctuation of the light source occurs, on the receiving side, the signal component cannot be output to a predetermined output port of the optical demultiplexer, or crosstalk to another channel occurs, and as a result, the normal There is a problem that the signal cannot be received. In order to solve such a problem, it is important to stabilize the oscillation frequency of each light source to a predetermined value in performing the optical frequency multiplex communication.

FIGS. 14 and 15 show a configuration of a conventional optical frequency multiplexed light source in which oscillation frequencies of a plurality of light sources are stabilized, and an operation principle thereof. This conventional example is disclosed in Japanese Unexamined Patent Publication No.
No. 1411, a plurality of semiconductor lasers 121 to 125 are used as light sources, the outputs of the respective lasers are multiplexed by a star coupler 21, and the multiplexed output is a Mach-Zehnder interferometer type optical filter (hereinafter referred to as “optical filter”). MZ filter ”)
33, the output is split by using an arrayed waveguide grating type optical splitter 44, and then received by using photodetectors 511 to 515 for each light source output. At that time, FIG.
As shown in the figure, the frequency intervals of the predetermined frequency of each light source are set so as to be arranged at equal intervals, and the cycle of the peak frequency of the transmittance of the MZ filter 33 having the sine wave characteristic on the frequency axis is set to the frequency interval of the light source. To match. Further, the oscillator 3 oscillates the transmission characteristic of the MZ filter 33 at a constant frequency.
4 and is modulated on the optical frequency axis. At this time, the outputs obtained from the photodetectors 511 to 515 are used as lock-in amplifiers 52 using the output of the oscillator 34 as a reference.
1 to 525. This allows the lock-in amplifier 5
Outputs of 21 to 525 become differential waveforms of the transmission signal, and M
It becomes zero at the peak frequency of the transmittance of the Z filter 33. Therefore, an error signal output corresponding to the frequency difference from the center frequency is obtained. By feeding back to the variable frequency power supplies 111 to 115 of each light source according to the error signal output, the oscillation frequency of the
The stabilization control can be performed to the peak frequency of the transmittance of the Z filter 33.

[0004]

However, in the structure of the prior art, the frequency (wavelength) interval of the light source can be stabilized, but the fluctuation of the center frequency of the MZ filter serving as a frequency reference is not taken into consideration, so that the light source is not considered. There was a problem that the absolute frequency could not be stabilized. Further, in the conventional example, the frequency interval of each light source is assumed to be constant, and the peak frequency interval of the transmittance of the MZ filter is set to coincide with the set frequency interval of the light source. It was not possible to stabilize the optical frequency multiplexed light source having an irregular interval.

An object of the present invention is to provide a multi-channel optical frequency stabilizing apparatus which can stabilize the absolute frequency of a light source and can be used even when the light source frequency interval is not constant. And

[0006]

According to a first aspect of the present invention, multiplexed light of output light from a plurality of light sources whose output optical frequencies are set to different values is input, and each of the plurality of light sources is An optical frequency discriminating means whose transmittance changes with respect to a deviation from the optical frequency set in the optical frequency discriminating means; an optical demultiplexing means for demultiplexing each output light of the light source from the output light of the optical frequency discriminating means; Optical frequency error detecting means for detecting an optical frequency error of each output light of the light source from the demultiplexed light of the optical demultiplexing means, and a feedback loop for controlling each output optical frequency of the light source according to the output of the optical frequency error detecting means (Hereinafter referred to as a “first feedback loop”), a frequency reference light source whose output optical frequency is stabilized, Discrimination error detection means for inputting the output light of the light source to the optical frequency discrimination means and detecting a discrimination error from the discrimination output, and control means for controlling the transmission center frequency of the optical frequency discrimination means according to the output of the discrimination error detection means (Hereinafter referred to as “second feedback loop”).

With this configuration, not only the frequency interval of the light source can be stabilized by the first feedback loop, but also the fluctuation of the transmission center frequency of the optical frequency discriminating means can be stabilized by the second feedback loop.
The absolute frequency of the light source can be stabilized.

There is provided means for modulating the transmission characteristic of the optical frequency discriminating means with a modulation signal, wherein the optical frequency error detecting means and the optical discriminating error detecting means respectively receive light splitting light of the optical demultiplexing means, Means for phase-locking the output of the light receiving means with a modulation signal.

Alternatively, the optical frequency discriminating means includes an optical filter having a structure in which complementary outputs can be obtained at two output ports, and the optical demultiplexing means includes an optical filter for each of the two output ports of the optical filter. of the output light is a configuration for demultiplexing, on at least one hand of the optical frequency error detection means and light discriminating the error detection means, light receiving means provided corresponding to the two output ports of the optical filter, the same Means for outputting a difference or ratio component between the two light receiving means in accordance with the optical frequency component of the light source may be included. According to this configuration, the light source frequency can be stabilized without modulating the optical frequency discriminating means. In this case, the optical demultiplexing means includes an arrayed waveguide grating type optical duplexer provided with a plurality of input / output ports facing each other, and a pair of opposed input / output ports of the arrayed waveguide grating type optical duplexer. Are connected to two output ports of an optical filter, and the light receiving means is connected to an input / output port other than the pair of input / output ports of the arrayed waveguide grating type optical splitter.

The light demultiplexing means may be formed as the same element as the means for multiplexing the output lights of the plurality of light sources.

According to a second aspect of the present invention, the multiplexed light of the output light of a plurality of light sources whose output optical frequencies are set to different values is input, and the optical frequency set for each of the plurality of light sources is input. Optical frequency discriminating means whose transmittance changes with respect to deviation from light, optical demultiplexing means for demultiplexing output light of each of the plurality of light sources from output light of the optical frequency discriminating means, and optical demultiplexing means Optical frequency error detecting means for detecting an optical frequency error of each output light of the plurality of light sources from the demultiplexed light, and a feedback loop for controlling each output optical frequency of the plurality of light sources according to the output of the optical frequency error detecting means In the multi-channel optical frequency stabilizing device provided with the above, the intervals of the optical frequencies adjacent to each other of the plurality of light sources are set differently, and the optical frequency discriminating means is set differently. Multi-channel optical frequency stabilizing device which comprises a periodic optical filter whose transmittance varies with a period equal to twice the common divisor of one or divisor thereof intervals of the light frequency is provided.

The second aspect is desirably implemented in combination with the first aspect, but may be implemented alone.

As the periodic optical filter, a Mach-Zehnder interferometer, a Fabry-Perot interferometer, or a periodic filter composed of a ring resonator can be used.

According to a second aspect of the present invention, a periodic optical filter is used as the optical frequency discriminating means, and its period or a half of the period is set to a common divisor of different frequency intervals of a plurality of light sources. Set to be one. When the period of the periodic optical filter is set to be a common divisor of the frequency interval of the light sources, in order to stabilize the output light frequency of each light source, the frequency region where the transmittance characteristics of the periodic optical filters are equal is considered. Is used to stabilize each light source.
Further, when the frequency interval of 周期 of the period of the periodic optical filter is set to be a common divisor of the frequency interval of the light source, not only the frequency region where the transmittance characteristics of the periodic optical filter become equal, but also A frequency region where the transmittance characteristic is inverted is also used.

As described above, by using a periodic optical filter whose transmittance changes at a period equal to one of the common divisors of the optical frequency interval or twice the common divisor, the optical frequencies are arranged at irregular intervals. In this case, each light source can be stabilized at a predetermined frequency.

Consider, for example, an optical frequency division multiplexing communication system that uses a 1.5 μm wavelength band as a light source. In such a system, when a dispersion-shifted fiber having a zero-dispersion wavelength near 1.5 μm is used as a transmission path, good transmission characteristics without wavelength degradation due to dispersion can be obtained even when a high-speed modulated signal is transmitted over a long distance. Obtainable. On the other hand, since the phase matching can be achieved between the light source wavelengths, a four-wave mixing based on the third-order nonlinearity of the refractive index of the fiber causes a frequency component corresponding to the signal frequency to be generated. Therefore, if the light source frequencies are arranged at equal frequency intervals, four-wave mixing light is generated at the same frequency as the signal frequency, resulting in crosstalk to the signal, causing deterioration in transmission characteristics.
To avoid such a situation, it has been proposed by Shibata et al. And Forzieri et al. To arrange light source frequencies at unequal intervals. Reference 1: Nobu Shibata, Ralph Brown, Robert Waltz, "Efficiency of Four-Wave Mixing Light in Frequency-Multiplexed Coherent Transmission Systems", IEICE Technical Report OQE86-8
3, September 29, 1986 Reference 2: F. Forghiei, RWTkach, ARChraplyvy and
D. Marcuse, "Reduction of four-wave-mixing cross ta
lk in WDM systems using unequally spaced channel
s ", OFC / IOOC '93 Technical Digest, pp.252-253, Fe
b.21-26, 1993 Among these, in the frequency array example shown in Reference 2, a frequency grid of 25 GHz is set, and a multiple value thereof is adopted as a frequency interval. In such a case, one cycle or 周期 cycle of the periodic filter is 25 G
Hz may be set.

According to a second aspect of the present invention, each frequency interval is a natural number times one cycle or one half cycle of the frequency filter. For this reason, the light source frequencies arranged at unequal frequency intervals can be stabilized while maintaining the optical frequency discrimination sensitivity.

[0018]

FIG. 1 is a block diagram showing a first embodiment of the present invention, and shows a plurality of light sources and a multi-channel optical frequency stabilizing device for stabilizing the optical frequency thereof.

The plurality of light sources 11 to 1n are set to output different optical frequencies from each other, and these output lights are multiplexed by the optical multiplexer 20 and output. The multiplexed light is used, for example, for optical frequency multiplexing communication, and a part of the multiplexed light is branched by the optical coupler 22 and input to the multi-channel optical frequency stabilizing device.

In the multi-channel optical frequency stabilizing device, the transmittance changes with respect to the deviation from the optical frequency set for each of the plurality of light sources 11 to 1n, and each frequency fluctuation of the multiplexed light is integrated. An optical frequency discriminator 30 for converting the output light from the optical frequency discriminator 30 into output light of each of the light sources 11 to 1n. From the demultiplexed light to the light source 11-1n
Error detection circuits 51 to 5n for detecting optical frequency errors (frequency fluctuations) of the respective output lights, and a first for controlling the respective output light frequencies of the light sources 11 to 1n according to the outputs of the error detection circuits 51 to 5n. A frequency reference light source having a stabilized output optical frequency in order to stabilize the center frequency fluctuation of the periodic optical filter constituting the optical frequency discriminator 30 and stabilize the absolute frequency of the light source. 31, an optical coupler 23 for multiplexing the output light of the frequency reference light source 31 with the multiplexed light of the output lights of the light sources 11 to 1n, and separating the output light of the frequency reference light source 31 from the output light of the optical frequency discriminator 30. An error detection circuit 5r for detecting the discrimination error, and a second feedback circuit for controlling the transmission center frequency of the optical frequency discriminator 30 according to the output of the error detection circuit 5r. And a croup. Light sources 11-1
n are set so that the intervals between adjacent optical frequencies are different from each other, and the optical frequency discriminator 30 outputs one of the common divisors of the differently set optical frequency intervals or 2 of the common divisor of the common divisor.
It is composed of a periodic optical filter whose transmittance changes at a period equal to twice.

In this embodiment, the interval between the optical frequencies output from the light sources 11 to 1n is stabilized by the first feedback loop, and the absolute frequency is stabilized by the second feedback loop. In addition, due to the transmittance characteristic of the optical frequency discriminator 30, even if the intervals between the adjacent optical frequencies of the light sources 11 to 1n are different, the oscillation light frequency can be stabilized.

FIG. 2 is a block diagram showing a second embodiment of the present invention. In this embodiment, the frequency reference light source 3
The output of the optical multiplexer 20 is multiplexed with the output light of each of the light sources 11 to 1n by the optical multiplexer 20, which is significantly different from the first embodiment. In this case, from the multiplexed output for communication, the optical filter 6
By 0, the frequency component ( _fr0 ) from the frequency reference light source 31
Is removed.

In the above embodiment, a star coupler may be used as the optical multiplexer 20 as in the conventional example, or a waveguide grating type multiplexer or the like may be used. Optical multiplexer 20
When a star coupler is used, the optical coupler 22 becomes unnecessary, and the other output of the optical multiplexer 20 can be a multiplexed output as in the conventional example. Further, as the optical demultiplexer 40, an optical filter, a multi-stage connection MZ filter, an arrayed waveguide grating type optical demultiplexer, or the like can be used.

FIG. 3 is a block diagram showing a third embodiment of the present invention. This embodiment is different from the above-described embodiment in that the multiplexing and demultiplexing of the output lights of the light sources 11 to 1n and the frequency reference light source 31 are realized by the same arrayed waveguide grating type optical multiplexer / demultiplexer 41. By properly designing the input and output wavelength characteristics of the arrayed waveguide grating type optical demultiplexer 41, different oscillation frequencies f ₁ ~f _n and f _r0 each other
When the output lights of the light sources 11 to 1n and the frequency reference light source 31 are input to the input ports a1 to an and ar of the arrayed waveguide grating type optical multiplexer / demultiplexer 41, these are multiplexed to the output side port b0. Is output. The multiplexed output is split into two outputs by the optical coupler 30, and one is discriminated by the optical frequency discriminator 30. Further, the output of the optical frequency discriminator 30 is input to the port a0 on the input side of the arrayed waveguide grating type optical multiplexer / demultiplexer 41 through the loopback optical path 70. Multiplexed light input to port a0 is the reciprocity of the transmission characteristics of the arrayed waveguide grating type optical demultiplexer 41, to the output side of the port b1~bn and br, optical frequency f ₁ ~f _n and f _r0 Is demultiplexed and output. These separated discrimination lights are respectively transmitted to the light sources 11 to 1n for the optical frequency discriminator 30 by the error detection circuits 51 to 5n.
Is detected, and an error detection circuit 5r detects a frequency error of the optical frequency discriminator 30 with respect to the frequency reference light source 31. The outputs of the error detection circuits 51 to 5n are fed back to the frequency variable means built in the light sources 11 to 1n, and the output of the error detection circuit 5r is fed back to the frequency characteristic variable means built in the optical frequency discrimination means 30. The optical frequencies 1n are stabilized at a predetermined absolute frequency.

FIG. 4 is a block diagram showing a fourth embodiment of the present invention. In this embodiment, the output light from the light sources 11 to 1n and the output light from the frequency reference light source 31 are not combined at once, but are provided by an optical coupler 25 provided separately from the arrayed waveguide grating type optical multiplexer / demultiplexer 41. The third embodiment differs from the third embodiment in that the output light of the frequency reference light source 31 is multiplexed with the multiplexed light of the light sources 11 to 1n.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described in detail below.

FIG. 5 is a block diagram showing a first embodiment. In this embodiment, semiconductor lasers 121 to 125 and
An optical multiplexer 20 for multiplexing the outputs, an optical coupler 22 for splitting the output into two outputs for extracting the multiplexed output and for stabilizing the frequency, and a frequency reference light source 3
1, combines the respective output light of the semiconductor laser 121 to 125 (the optical frequency _{f 1, f 2, f 3} , f 4, f 5) and frequency reference light source 31 outputs light (optical frequency f _r0) An optical coupler 23, an MZ filter 33 for optical frequency discrimination provided with an MZ filter tuning electrode 32, an oscillator 34 for modulating the MZ filter 33, and an MZ filter 3
Optical splitter 41 for splitting the optical frequency multiplexed signal transmitted through 3
And photodetectors 511 to 516 for receiving the demultiplexed signal components, and lock-in amplifiers 521 to 5 for extracting fluctuation components of each optical frequency from each output of the photodetectors 511 to 516 using the signal of the oscillator 34 as a reference signal.
26, variable frequency power supplies 111 to 115 for changing the respective bias currents or temperatures of the semiconductor lasers 121 to 125 based on the outputs of the lock-in amplifiers 521 to 525, and the MZ filter 33 based on the output of the lock-in amplifier 526. An adder 35 detects a frequency fluctuation component, adds the output of the oscillator 34, and applies a bias to the MZ filter tuning electrode 32.

The first feature of this embodiment is that the MZ filter 3
A first feedback loop for stabilizing the frequency interval between the plurality of semiconductor lasers 121 to 125 using the periodic frequency characteristic of No. 3, and a second feedback for stabilizing the MZ filter 33 by the frequency reference light source 31. It is provided with a loop. Here, the oscillation frequency of the frequency reference light source 31 and another frequency multiplexed light source (semiconductor laser 121)
To 125) are also set in the MZ filter 3
3 is a natural number multiple of the cycle of the frequency transmission characteristic or a natural number multiple of a half cycle. As the frequency reference light source 31, for example, a light source whose frequency is stabilized by an absorption line of HCN or C ₂ H ₂ existing in a 1.5 μm wavelength band is used. In such a light source, the transmission characteristics of the MZ filter 33 are stabilized, and not only the frequency interval can be stabilized, but also the absolute value of the light source frequency can be stabilized.

A second feature of this embodiment is that the frequency arrangement set for the semiconductor lasers 121 to 125 is unequally spaced, and the period of the periodic transmission characteristic of the MZ filter 33 is unequally spaced. Common divisor of the frequency interval of
It is set to double. That is, the frequency intervals of the unequally spaced arrangement are a natural number multiple of the cycle of the frequency transmission characteristic of the MZ filter 33 or a natural number multiple of a half cycle. As a result, even if the frequency arrangement is unequal, each can be stabilized.

FIG. 6 shows the relationship between the light source frequency arrangement and the transmission characteristics of the optical frequency discriminator (MZ filter 33). Here, an MZ filter 3 used as an optical frequency discriminator is used.
の of the periodic transmission characteristic of the semiconductor laser 12
Optical frequency f ₁₀ which is set to _{1~125, f 20, f 30,} f
_40, and a common divisor of the frequency spacing f _50, the frequency interval between the peak frequency and the bottom frequency of the MZ filter 33 as a frequency grid Delta] f _g, each frequency interval 3Δ
f _g , 6Δf _g , 5Δf _g , and 2Δf _g . Further, the reference light frequency _fr0 from the frequency reference light source 31 is represented by _fr0
= And f ₅₀ + Δf _g. The transmission characteristic of the MZ filter 33 can be stabilized by matching one peak frequency or bottom frequency of the transmission characteristic of the MZ filter 33 with the reference light frequency _fr0 . Further, the output light frequency f ₁ of the semiconductor lasers 121 to 125 is set at the peak frequency or the bottom frequency of the MZ filter 33 whose transmission characteristics are stabilized.
By matching f ₂ , f ₃ , f ₄ , f ₅ respectively,
Not only the frequency interval but also the absolute value of the frequency can be stabilized.

The operation principle of this embodiment will be described in more detail. Semiconductor lasers 121 to 1 to be frequency stabilized
The output light of each of the light source 25 and the frequency reference light source 31 is multiplexed by the optical couplers 22 and 23 and input collectively to the MZ filter 33. Optical frequency f ₁ ~f ₅ respective predetermined frequency f ₁₀ ~f ₅₀ of the semiconductor laser 121 to 125
Is set in advance in the vicinity of. On the other hand, MZ Furuta 3
One peak frequency f _r of the transmission characteristics of 3 is set in the vicinity of the frequency reference source f _r0. Further, by supplying the output of the oscillator 34 to the MZ filter tuning electrode 32, the transmission characteristic of the MZ filter 33 is minutely modulated. The MZ filter tuning electrode 32 is a thin film heater formed on the waveguide of the MZ filter 33, and generates heat by applying a bias current thereto, and shifts the center frequency of the filter transmission characteristics by a thermo-optic effect. it can. The output of the MZ filter 33 is split by the optical splitter 40 and received by the photodetectors 511 to 516. These outputs are connected to lock-in amplifiers 521 to 526.
And performs lock-in detection using the signal of the oscillator 34 as a reference signal. The output of the lock-in amplifier 521 to 526, as shown in FIG. _{6, f 10, f 20,} f 30, f 40, f 50
And f _r0 are set to zero output, and the frequency shift amount δf
_{i = f i -f i0 (i} = 1,2,3,4,5, r) becomes the error signal output corresponding to the. In particular, f _i0 where δf _i is sufficiently small
, An error signal output proportional to δf _i is obtained,
This output can also be used as an oscillation frequency monitor for each light source. Of these error signals, the lock-in amplifier 5
The output of the MZ filter 3 from the reference optical frequency _fr0
3 corresponds to the displacement of the peak frequency f _r, which by feeding back the MZ filter tuning electrode 32 through the adder 35, it is possible to stabilize the reference optical frequency peak frequency of the MZ filter 33. Further, the outputs of the lock-in amplifiers 521 to 525 are fed back to the bias currents or operating temperatures of the semiconductor lasers 121 to 125 through variable frequency power supplies 111 to 115, respectively, so that the semiconductor lasers 121 to 125
Can be stabilized to a predetermined absolute frequency.

FIG. 7 is a block diagram showing a second embodiment. In the first embodiment, the output of the frequency reference light source 31 is combined with the output light of the semiconductor lasers 121 to 125 and input to the MZ filter 33 at a time. The multiplexed light from 125 is MZ
Input to port C1 of filter 33, frequency reference light source 3
By inputting the output of port 1 to port C2,
The multiplexed light is obtained at port C3. With this configuration, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 becomes unnecessary, and the number of components can be reduced.

FIG. 8 is a block diagram showing a third embodiment. This embodiment differs from the first embodiment in the signal path of the output light of the frequency reference light source 31. That is, the output of the frequency reference light source 31 is input to the port C4 of the MZ filter 33 via the optical isolator 37, and the light output from the port C2 is received by the photodetector 516. The output light of the semiconductor laser 121 to 125 (the optical frequency f ₁ ~f ₅₎
Of the output light of the frequency reference light source 31 (optical frequency f _r0 )
Since the propagation directions in the Z filter 33 are different, only the frequency component from the frequency reference light source 31 is input to the photodetector 516, and interference is prevented. Also, the optical demultiplexer 40
Does not require a port for frequency reference light demultiplexing, and can simplify the configuration. The principle of frequency stabilization is the same as in the first and second embodiments.

FIG. 9 is a block diagram showing a fourth embodiment. In this embodiment, without modulating the MZ filter 33, a difference or a logarithmic difference component of each light source frequency component obtained from the two output ports C3 and C4 (a component obtained by converting to a logarithm and taking a difference, It is different from the first embodiment in that an error signal component from a predetermined frequency is obtained by using the same as that of obtaining the two ratios). The configuration of this embodiment is similar to that of the first embodiment.
And output to the port C1 of the MZ filter 33. The multiplexed light that has been subjected to optical frequency discrimination by the MZ filter 33 is output to ports C3 and C1, and is demultiplexed by the optical demultiplexers 41 and 42, respectively. The demultiplexed outputs of the optical demultiplexer 41 are photodetectors 511 to 516, and the demultiplexed outputs of the optical demultiplexer 42 are photodetectors 531 to 53.
6 and input to a differential or logarithmic differential amplifier 514 to 546 for each frequency component to obtain a difference or a logarithmic difference. The output of the differential or logarithmic differential amplifier 546 corresponding to the frequency error between the frequency reference light source 31 and the MZ filter 33 is fed back to the MZ filter tuning electrode 32, and the frequency characteristics of the MZ filter 33 are Stabilizes to the reference light frequency. Also, the outputs of the differential or logarithmic differential amplifiers 541 to 545 are fed back to the variable frequency power supplies 111 to 115 to stabilize the frequency of the semiconductor lasers 121 to 125.

FIG. 10 shows the relationship between the frequency arrangement of the semiconductor lasers 121 to 125 and the MZ filter 33 and the difference between the outputs of the MZ filter 33 (error signal output). When a signal is input to port C1 of MZ filter 33, port C1
The output characteristic of port 3 and the output characteristic of port C4 have a complementary relationship, and ideally have the same output level at a frequency at which the transmittance becomes half the peak. A frequency satisfying such a condition (here, referred to as “と い う output frequency”) periodically exists at the grid frequency Δf _g . Therefore, the oscillation frequencies f ₁₀ , f ₁₀ set for the semiconductor lasers 121 to 125 are
₂₀ , f ₃₀ , f ₄₀ , f ₅₀ and the reference light frequency f _r0 of the MZ filter 33 are set to be １ ／ of this output frequency. To this end, first, the semiconductor lasers 121 to 12
5 each one half the output frequency f _r of the oscillation frequency f ₁ ~f ₅ and MZ filter 33 is set in the vicinity of f ₁₀ ~f _50, f _r0 advance, respectively. In this case, the difference between the outputs of the two ports C3, C4 of the MZ filter 33, as shown in the lower part of FIG. 10, f ₁₀ ~f _50,
the f _r0 is set to zero output, frequency error _{δf i = f i -f i0 (} i
= 1, 2, 3, 4, 5, r).

This embodiment is characterized in that it is not necessary to modulate the MZ filter 33 as compared with the first to third embodiments. Therefore, it is not necessary to use the MZ filter tuning electrode 32 having both the purpose of modulation and the frequency stabilization, and the frequency can be stabilized by feedback-controlling the temperature of the MZ filter 33.

FIG. 11 is a block diagram showing a fifth embodiment. In this embodiment, the semiconductor lasers 121 to 1
25 and the frequency arrangement of the MZ filter 33, and optical frequency discrimination of the combined light through the MZ filter
3, the configuration for outputting to C4 is the same as that of the fourth embodiment.
This embodiment is different from the fourth embodiment in that a 7-input, 7-output arrayed-waveguide grating type optical demultiplexer 44 is used as an optical demultiplexer. With this configuration, the functions of the two optical demultiplexers 41 and 42 in the fourth embodiment can be put together,
The number of parts can be reduced. The arrayed waveguide grating type optical demultiplexer 44 has left-right reciprocal demultiplexing characteristics. By inputting the output light from the port C3 of the MZ filter 33 to the port b1, the ports a2, a3, a4, a5, a6, The frequency components f ₁ , f ₂ , f ₃ , f ₄ , f ₅ and f _r0 are output to a7. Port C of the MZ filter 33
By inputting the output light of port 4 to port a1, port b
2, b3, b4, b5, b6, b7 have f ₁ , f
_2, the frequency component of f _3, f _4, f ₅ and f _r0 is output. These split light beams are converted into photodetectors 511-51.
6, 531 to 536, and stabilizes the frequencies of the semiconductor lasers 121 to 125 and the MZ filter 33 as in the fourth embodiment.

FIG. 12 is a block diagram showing a sixth embodiment. In this embodiment, the output light of the frequency reference light source 31 is input to the port C2 of the MZ filter 33 as shown in FIG.
Is different from the fourth embodiment shown in FIG. In this configuration, as in the second embodiment shown in FIG. 7, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 becomes unnecessary, and the number of components can be reduced.

Although the optical demultiplexers 41 and 42 are shown separately in FIG. 12, an arrayed waveguide grating type optical demultiplexer having input / output terminals of twice or more the number of optical multiplexes is used. Thus, due to its periodicity, one optical demultiplexer 41, 42
Can play a role.

FIG. 13 is a block diagram showing a seventh embodiment. In this embodiment, the output light of the frequency reference light source 31 is input to the port C2 of the MZ filter 33 as shown in FIG.
This is different from the fifth embodiment shown in FIG. In this configuration, similarly to the second embodiment shown in FIG. 7 and the sixth embodiment shown in FIG. 12, the optical coupler 23 for multiplexing the output light of the frequency reference light source 31 becomes unnecessary, and the number of parts is reduced. Can be reduced.

In the sixth embodiment and the seventh embodiment, the MZ filter tuning electrode 32 is feedback-controlled to stabilize the frequency of the MZ filter 33.
The temperature of the MZ filter 33 may be controlled, and the temperature control circuit may be feedback-controlled.

In the above embodiments, specific examples of the first and second embodiments have been described. However, similar configurations are possible in the third and fourth embodiments.

[0043]

As described above, the multi-channel optical frequency stabilizing device of the present invention controls the frequency interval of a plurality of light sources by the period of the transmittance change of the periodic optical filter, By stabilizing the transmission center frequency of the filter, not only the interval between the optical frequencies of the light source but also the absolute frequency can be stabilized.

Also, by setting the period of the transmittance change of the periodic optical filter or a value that is a natural number multiple of a half period thereof to be the frequency interval of the light source, the frequency intervals of the light source are arranged at irregular intervals. In this case, the oscillation frequency interval of each light source can be stabilized. Therefore, it is possible to improve the frequency stability when performing frequency multiplex transmission while avoiding generation of four-wave mixing light.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the present invention.

FIG. 4 is a block diagram showing a fourth embodiment of the present invention.

FIG. 5 is a block diagram showing a first embodiment.

FIG. 6 is a diagram showing a relationship between a light source frequency arrangement and transmission characteristics of an MZ filter.

FIG. 7 is a block diagram showing a second embodiment.

FIG. 8 is a block diagram showing a third embodiment.

FIG. 9 is a block diagram showing a fourth embodiment.

FIG. 10 is a diagram showing a relationship between a frequency arrangement of a semiconductor laser and an MZ filter and a difference between outputs of the MZ filter.

FIG. 11 is a block diagram showing a fifth embodiment.

FIG. 12 is a block diagram showing a sixth embodiment.

FIG. 13 is a block diagram showing a seventh embodiment.

FIG. 14 is a block diagram showing a conventional example.

FIG. 15 is a diagram showing the operation principle of a conventional example.

[Explanation of symbols]

 11 to 1n Light source 111 to 115 Variable frequency power supply 121 to 125 Semiconductor laser 20 Optical multiplexer 21 Star coupler 22 to 25 Optical coupler 30 Optical frequency discriminator 31 Frequency reference light source 32 NZ filter tuning electrode 33 MZ filter 34 Oscillator 35 Adder 40, 42, 43 Optical demultiplexer 41 Arrayed waveguide grating type optical multiplexer / demultiplexer 44 Arrayed waveguide grating type optical duplexer 51-5n, 5r Error detection circuits 511-516, 531-536 Photodetectors 521-526 Lock-in Amplifiers 541 to 546 Differential or logarithmic differential amplifier 60 Optical filter

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI H04J 14/02 (56) References JP-A-8-51411 (JP, A) JP-A-3-21936 (JP, A) JP-A-3-78335 (JP, A) JP-A-3-64084 (JP, A) JP-A-8-251105 (JP, A) JP-A-63-45877 (JP, A) (58) Fields investigated Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H01S 5/0687 H04J 14/00-14/08

Claims (6)

(57) [Claims] 1. A multiplexed light of output lights of a plurality of light sources whose output optical frequencies are set to values different from each other is input, and each of the plurality of light sources is transmitted with respect to a deviation from an optical frequency set for each of the plurality of light sources. Optical frequency discriminating means for changing the rate; optical demultiplexing means for demultiplexing output light of each of the plurality of light sources from output light of the optical frequency discriminating means; and An optical frequency error detecting means for detecting an optical frequency error of each output light of the light source, and a feedback loop for controlling an output optical frequency of each of the plurality of light sources according to an output of the optical frequency error detecting means. In the channel optical frequency stabilizing device, a frequency reference light source whose output optical frequency is stabilized, and an output light of the frequency reference light source is input to the optical frequency discriminating means and discrimination is performed. A discriminating error detecting means for detecting a discriminating error from a force, according to an output of the discriminating error detecting means, by making one of a peak frequency or a bottom frequency of transmission characteristics of the optical frequency discriminating means coincide with a reference optical frequency, Control means for matching the output light frequency of the light source with the peak frequency and the bottom frequency of the detection means. 2. The interval between optical frequencies adjacent to each other of the plurality of light sources is set differently, and the interval between optical frequencies is a natural number times a half cycle of the periodic frequency transmission characteristic of the optical frequency discriminating means. Claim 1
A multi-channel optical frequency stabilizing device as described in claim 1. 3. A device for modulating a transmission characteristic of the optical frequency discriminating means with a modulation signal, wherein the optical frequency error detecting means and the optical discriminating error detecting means respectively receive the split light of the optical demultiplexing means. 3. The multi-channel optical frequency stabilizing device according to claim 1, further comprising: a light receiving unit that performs phase locked detection of an output of the light receiving unit using the modulation signal. 4. The optical frequency discriminating means includes an optical filter having a structure in which complementary outputs are obtained at two output ports, and the optical demultiplexing means includes an output light of each of two output ports of the optical filter. A light source provided on at least one of the optical frequency error detecting means and the optical discrimination error detecting means, the light source being provided corresponding to two output ports of the optical filter, and the same light source. And means for outputting a difference or ratio component between the two light receiving means in response to the optical frequency component of (1).
A multi-channel optical frequency stabilizing device as described in claim 1. 5. The optical demultiplexing means includes an arrayed waveguide grating type optical duplexer provided with a plurality of input / output ports facing each other, and a pair of opposed sets of the arrayed waveguide grating type optical duplexer. 5. The input / output port is connected to two output ports of the optical filter, and the light receiving means is connected to an input / output port other than the set of input / output ports of the arrayed waveguide grating type optical demultiplexer. A multi-channel optical frequency stabilizing device as described in claim 1. 6. The light demultiplexing means is formed as the same element as the means for multiplexing the output lights of the plurality of light sources.
6. The multi-channel optical frequency stabilizing device according to any one of items 5 to 5.